The present invention relates to a memory system having a fast and stable sense amplifier with a high sensitivity, and, more particularly, to a monolithic memory system which makes use of field effect transistors as circuit elements.
A recently developed monolithic memory comprises a plurality of memory cells, each including a field effect transistor and a capacitance, and a differential sense amplifier having a flip-flop circuit constructed with field effect transistors. This sense amplifier differentially amplifies the voltages of a pair of data lines and detects the signal stored in a selected one of the memory cells. The flip-flop circuit has a latch node which is precharged to a predetermined voltage before the flip-flop circuit begins its amplifying operation. The data lines are also precharged to nearly the same voltage. Before the amplifying operation, the transistors which constitute the flip-flop circuit are in off states. Under that condition, the signal stored in one of the memory cells is read out, and the voltage of one of the data lines changes in response to the read out signal. Then, a discharging field effect transistor connected to the latch node of the flip-flop circuit is turned on.
As a result, the node voltage is lowered to 0 volts and the flip-flop circuit begins its amplifying operation. The switching of the discharging field-effect transistor is controlled by a voltage pulse supplied by a pulse circuit located at the periphery of the memory cell area. When the voltage pulse becomes higher than the threshold voltage V.sub.T of the field-effect transistor, the transistor becomes conductive. The transistor must become conductive after the signal stored in a memory cell is read out onto a corresponding data line.
One problem with this prior art memory system is that it often fails to operate normally when the threshold voltage V.sub.T is small for reasons discussed below.
In order to develop a memory with a higher packing density, it is necessary to reduce the sizes of the transistors used in the memory. It is also necessary to reduce the power supply voltage V.sub.DD which is provided to the transistors, in order to avoid the dielectric breakdown of the transistors due to scaling down the component size to achieve the higher packing density. At the same time, it is necessary to reduce the threshold voltage V.sub.T of the transistors in order to maintain a high speed operation. The switching speed of a field effect transistor is roughly proportional to (V.sub.G -V.sub.T).sup.n, where V.sub.G is a gate voltage and n is a an experimentally determined number among 1.0 and 2.0. If the gate voltage V.sub.G is assumed to be nearly equal to the power supply voltage V.sub.DD, a lower threshold voltage V.sub.T is desirable when a lower supply voltage is used, in order to maintain a high speed operation.
If the discharging field effect transistor connected to the latch node of flip-flop circuit as discussed above has a lower threshold voltage V.sub.T, the transistor is apt to become conductive even when it should be non-conductive, due to voltage drift of the voltage pulse provided to the gate thereof. As a result, the latch node of the flip-flop circuit begins to discharge during a period when it is not supposed to, and the circuit performs its amplifying operation, discharging the data lines. The output signal of the flip-flop circuit at the period when the amplified signal should be detected is then lowered because of the above-mentioned malfunction.
The voltage drift of the voltage pulse which causes this malfunction may be introduced because of several reasons. One of these reasons stems from the fact that pulse circuit which provides the voltage pulse is connected to a common ground line and provides a voltage pulse the level of which is dependent on the voltage of the common ground line at the point where the pulse circuit is connected thereto. The resistance value of the ground line is not negligible and several other peripheral circuits are also connected to this ground line. Each of these peripheral circuits introduces a current flow through the ground line while the circuit operates. As a result, the voltage of the ground line drifts depending on how many of the peripheral circuits are operating. Therefore, the discharging transistor begins to discharge during an undesirable period due to this voltage drift, if the threshold value V.sub.T is low.